Re: [Phys-L] just for fun?

[John Denker <jsd@av8n.com>]

Let's start with a parable.

You may have heard that according to the laws of physics, if 
you add heat energy to something, it gets hotter.  So let's
do the experiment.  In your home this weekend, set up a 
heat-lamp so that it shines on the heating-system thermostat,
or set up a small electric heater so that it has the same
effect.  We want a *small* amount of local temperature rise,
only a few degrees.  Maintain it for an hour or two.  Observe 
whether the house as a whole gets hotter or colder.

The moral of the story is:  Feedback loops are complicated.
Guessing and hand-waving will not get the right answer.

We know that the earth's climate system has all sorts of
feedback loops.  Otherwise there wouldn't be ice ages.

I mention this because on 01/03/2014 03:09 PM, Richard Tarara
wrote:

> The plots that have been shown consistently over the past dozen years
> from the ice-core data show a several hundred year lag!

OK...........

> To be sure, this may be due to the fact that the temperature changes 
> in this data have been mostly due to glaciation and de-glaciation 
> with CO2 effects that are relatively complicated as detailed in the 
> reference given earlier.
>
> http://www.realclimate.org/index.php/archives/2012/04/unlocking-the-secrets-to-ending-an-ice-age/

OK, so far so good.  I don't see a problem here.

> My complaint lies with the likes of Al Gore who centrally used this
> data to PROVE that CO2 was causing the current warming and telling
> everyone they were complete idiots if they couldn't see this from his
> huge plot.

The scientific fact is that there are a lot of moving parts.
There is a /chain/ of causation.

   A --> B --> C --> D --> E --> F --> G et cetera.

So, C does not cause B ... but C *does* cause D directly,
and leads to E, F, and G indirectly.  (This is all without
accounting for the various feedback loops.)

Now suppose we introduce a humongous perturbation (XXX)
that looks like additional C:


   A --> B --> C --\
                    D --> E --> F --> G et cetera.
             XXX --/

Now C still does not cause B ... but C still does cause
D directly, and still leads to E, F, and G indirectly.

Now in reality there are feedback loops, such that E
can act back on B:
          ________________
         |                |
         v                |  
   A --> B --> C --\      |
                    D --> E --> F --> G et cetera.
             XXX --/
    
In such a case, unsophisticated notions of cause-and-effect
go out the window, as illustrated by the thermostat parable.

In particular, if this is a /stable/ negative feedback loop,
then the variation in C will be roughly 180 degrees out of 
phase with the variation in XXX.  If you were expecting C 
to lead D by 5 degrees, instead you will see something like
a 175 degree lag.  Alternatively, if this is a positive 
feedback loop, then nobody can guarantee anything, not 
without a much more detailed analysis.

For more on this, try
  https://www.google.com/search?q=feedback+loop+perturbation+analysis

If this is too complicated for your audience, go back to
the thermostat experiment.  It is reeeeeally easy for a
feedback system to produce results that seem paradoxical
to the naïve observer.

On the third hand, a certain amount of complexity is part
of the story.  There are some simple questions that do not
have a simple answer.  This is why "global warming" is the
asking the wrong question.  Instead, "climate change" is
a much better question, and that's because each little
region of the earth has its own climate ... and not all
of them are changing in the same way.

Also, it is a gross fallacy to use simple "pattern matching" 
as a substitute for reasoning.  You cannot simply look into
the fossil record for raw data that "looks like" the present
situation, because the present situation is unprecedented.

Instead you have to model the physics.  The physics!  Maybe
there are no clear examples in the past where the CO2 levels
directly drove Antarctic melting ... but that does *not* 
mean that current unprecedented levels of CO2 are incapable 
of doing so.